1. Field of the Invention
The present invention relates to a base station, and a scrambling code setting method for a CDMA cellular system, using a code division multiple access method, wherein a user shares the same frequency band with other users.
2. Description of the Related Art
In a communication system, wherein two or more communication waves share the same frequency band using the code division multiple access method, each communication wave is distinguished by a spread code that is a random code sequence. System efficiency is the higher, if a detection property (autocorrelation property) and a discernment property between different spread codes (cross-correlation property) are the better. For this reason, not all the random code sequences available by sequence length are used, but only random code sequences that have a necessary property are used. Accordingly, when high system efficiency is required, the number of spread codes that are available for assignment becomes smaller than the number of communication waves that a communication system assumes as being simultaneously required.
Especially, for the spread codes that are used by downlinks of a wideband CDMA cellular system, sign orthogonality is required in addition to the high detection and discernment properties so that high quality communications are realized at low transmission power by reducing mutual interference between the spread codes that are different. For this reason, the number of spread codes that can be assigned to the downlink becomes further smaller than to an uplink. These few spread codes are repeatedly used by all the cells for effective usage, with a cell-unique code being superimposed in order to prevent interference between the cells.
In the wideband CDMA cellular system, spread codes that actually spread user data are called channelization codes, and codes that are superimposed by each cell for cell interference prevention are called scrambling codes.
The number of the scrambling codes for distinguishing a cell is not infinite, either. The number is specified as 512 for the wideband CDMA cellular system. Accordingly, in order to prevent interference, it is necessary to select the scrambling codes such that the same code is not repeatedly used in a short distance. Conventionally, scrambling codes are manually selected by a radio network designer so that the same scrambling code is not assigned to base stations that are close to each other. Specifically, a radio wave reaching area of each base station is estimated using a computer into which positions of the base stations, and equipment and antenna configuration are entered, and the scrambling codes are assigned such that the same code is not assigned to base stations, the radio wave reaching areas of which overlap. For this reason, when a new base station is to be installed, the radio network designer has to perform this set up process again.
Further, when scrambling codes are assigned based on computer simulation, an error can arise in actual operations, which causes interference. In this case, it is necessary to perform setting up again, further consuming great time and effort of the radio network designer and an on-site surveyor.
In order to solve the problem described above, a method of automatically assigning a scrambling code for a downlink of a base station is disclosed (for example, Patent Reference 1). According to this method, first, a base station of a wideband CDMA cellular system receives a downlink signal of an adjacent base station, identifies its control channel, and performs decoding such that the scrambling code already used by the downlink of the adjacent base station is identified. Then, based on the result, a control unit of the base station automatically determines a downlink scrambling code to be used by the base station.
This method is characterized in that the base station receives the downlink of the adjacent base station. Accordingly, the base station includes
a filter for receiving the downlink frequency band,
a decoding unit, and
a processing unit for identifying the control channel of the downlink, and for detecting a scrambling code and a channelization code. Then, the base station sets up a scrambling code, the number of which is higher than the scrambling code used by the adjacent base station. In this way, assigning the same scrambling code to two adjacent base stations is avoided.
[Patent reference 1] JPA 2002-218528